The obesity epidemic is claiming children at ever younger ages, and the latest research adds to the evidence that weight issues may begin as soon as birth.

In a study published in the journal Archives of Disease in Childhood, researchers found that babies born by cesarean section were more than twice as likely to be obese by age 3 as those born vaginally.

The study involved 1,255 children born in the Boston area between 1999 and 2002, whose mothers agreed to provide their prenatal and gestational weight information, as well as height and weight measurements for their babies at birth and then until they reached 3 years old.

About 23% of the babies were born by c-section. Of these children, 15.7% were obese by age 3, compared with 7.5% of children born vaginally.

The association remained even after the scientists adjusted for other factors that could influence child obesity, including the mother’s pregnancy weight and the birth weight of the baby. In fact, when the scientists looked at the mothers according to their BMI, the relationship between cesarean delivery and higher child obesity rates remained even among mothers who were of normal weight, suggesting that maternal weight was not influencing children’s obesity at age 3.

“One in 3 babies in the U.S. is now delivered by cesarean,” says the study’s lead author, Dr. Susanna Huh, director of the growth and nutrition program at Children’s Hospital in Boston and an assistant professor of pediatrics at Harvard Medical School. “Mothers need to be aware that our study suggests that children delivered by cesarean section may be at higher risk of obesity.”

Why? It might have something to do with how delivery affects the makeup of bacteria that populate infants’ digestive tracts. During their first year of life, infants born by c-section tend to have higher quantities of Firmicutes bacteria in their guts. Studies show that these bacteria can influence weight: obese adults tend to have more Firmicutes than any other species of gut bacteria, and research in mice finds that reducing the amount of gut Firmicutes lowers animals’ risk of weight gain.

Infants born vaginally are exposed to their mother’s vaginal microbes, while those born by c-section are not. Further, it’s possible that the antibiotics used during Cesarean delivery also influences the population of infants’ gut bugs, which can in turn impact babies’ weight.

These differences prompted Huh and her colleagues to explore whether mode of delivery had any long-term influence on children’s weight. Such differences have previously been linked to other childhood conditions. For example, babies born by cesarean section have a higher risk of asthma than vaginally delivered kids, also because of the different microbes that these babies acquire during birth and in their first year of life.

Huh acknowledges that further study is needed to understand the mechanisms that underlie the association between c-section and obesity, but she says expectant mothers who are considering elective c-sections over vaginal delivery may want to think about whether their birth plan will have long-lasting implications for their infants’ health. “Mothers considering cesarean delivery in the absence of any medical indication should be counseled that their children could be at higher risk of obesity,” says Huh.

Alice Park is a writer at TIME. Find her on Twitter at @aliceparkny. You can also continue the discussion on TIME’s Facebook page and on Twitter at @TIME.